Close the post-selection loophole in the quantum-SWITCH QKD implementation

Close the post-selection loophole in the photonic quantum key distribution implementation based on a quantum SWITCH by replacing the probabilistic linear-optical entangling gates with deterministic photon–photon interactions, such as cavity-QED or other matter-mediated interaction schemes.

Background

The demonstrated quantum key distribution scheme uses pre-entanglement between the system and ancilla photons and post-selected linear-optical gates to perform path-coherence-preserving measurements inside the quantum SWITCH. Because the entangling operations succeed only probabilistically, the experiment does not provide a fully deployable or proven-secure QKD protocol.

The unresolved issue is the post-selection loophole: successful detection events do not by themselves guarantee that all required interactions occurred successfully, which can undermine security. The paper identifies deterministic photon–photon interactions, including cavity-QED or other matter-mediated interfaces, as the type of technology needed to remove this limitation.

References

Consequently, the protocol relies on post-selection, leaving the post-selection loophole open. Closing this loophole would require replacing the probabilistic linear-optical entangling gates with deterministic photon--photon interactions enabled by nonlinear light--matter interfaces, such as cavity QED or other matter-mediated interaction schemes.

— Experimental Quantum Key Distribution in an Indefinite Causal Order  (2608.13561 - Valibouse et al., 13 Aug 2026) in Section 3, subsection “Setup for QKD”